Fluorination process

ABSTRACT

The invention relates to a process for preparation of a compound of formula (I): 
                         
R 1  is selected from C 1-6 alkyl, C 2-6 alkenyl, and C 2-6 alkynyl;
 
which comprises:
 
(i) reaction with fluoride, suitably [ 18 F]fluoride, of a corresponding compound of formula (II):
 
                         
wherein R 2  is selected from hydrogen, C 1-10 alkyl, C 1-10 haloalkyl, C 6-14 aryl, C 6-14 arylalkyl, —(CH 2 CH 2 O) q —CH 3  wherein q is an integer of from 1 to 10;
 
R 1  is as defined for the compound of formula (I); and
 
R 3  is a leaving group. Certain novel precursors of formula (II) and radiopharmaceutical kits containing such precursors are also claimed.

The present invention relates to novel processes for fluorination,particularly [¹⁸F]fluorination of certain aromatic compounds, and to thenovel precursors used in the process. The invention has particularutility for a class of benzothiazole derivatives which are known as invivo imaging agents.

Fluorination of aromatic compounds may be performed via electrophilicreaction with molecular fluorine, however, electrophilic fluorination ofaromatic compounds with fluorine is generally a poor and non-selectivemethod. Different electrophilic fluorinating reagents prepared frommolecular fluorine, such as CH₃COOF “AcOF” have been developed butsuffer several drawbacks. For [¹⁸F]radiofluorination, the harshconditions, and poor availability of electrophilic [¹⁸F]fluorinationmethods, and the low specific activity of the products obtained meanthat it is not a favoured approach for commercial production of[¹⁸F]-labelled products. Nucleophilic fluorination methods, usingfluoride are more commonly used. [¹⁸F]Fluoride is a more widelyavailable reagent than the electrophilic reagents and the productsobtained are of higher specific activity which is advantageous in thefield of in vivo imaging. Nucleophilic fluorination of aromatic rings,particularly those which are electron rich can be problematic. Forexample, nucleophilic fluorination of anilines, where the amino groupadds to electron density of the aromatic ring is difficult to perform.The present invention provides a precursor suitable for nucleophilicfluorination to give a fluorinated aniline, which combines a number ofbeneficial effects including improved activation of the aromatic ring tofluorination, few steric effects on the fluorination reaction, andfacile conversion to the fluorinated aniline product.

According to the invention, there is provided a process for preparationof a compound of formula (I):

wherein phenyl ring A is optionally substituted by 1 to 4 substituents;R¹ is selected from C₁₋₆alkyl, C₂₋₆alkenyl, and C₂₋₆alkynyl;which comprises:(i) reaction of a corresponding compound of formula (II):

wherein ring A is optionally substituted as defined for the compound offormula (I);R² is selected from hydrogen, C₁₋₁₀alkyl, C₁₋₁₀haloalkyl, C₆₋₁₄aryl,C₆₋₁₄arylalkyl, —(CH₂CH₂O)_(q)—CH₃ wherein q is an integer of from 1 to10;R¹ is as defined for the compound of formula (I); andR³ is a leaving group;with fluoride to give a compound of formula (III)

wherein R¹ and R² are as defined for the compound of formula (I) andphenyl ring A is substituted as defined for the compound of formula (I);followed by step (ii) and optionally step (iii) in any order(ii) conversion of group —C(O)R² to hydrogen, suitably by hydrolysis(iii) removal of any further protecting groups.

Phenyl ring A is optionally substituted by 1 to 4 organic substituentsfor example selected from fluoro, chloro, bromo, iodo, cyano, nitro, —R,—OR, —OC(O)R, —C(O)R, —SR, —NR₂, —C(O)NR₂ wherein R at each occurrenceis selected from C₁₋₆alkyl, C₂₋₆alkenyl, C₂₋₆alkynyl,C₁₋₆alkoxy-C₁₋₆alkyl, C₁₋₆haloalkyl, C₂₋₆haloalkenyl, C₂₋₆haloalkynyl,C₁₋₆haloalkoxy-C₁₋₆alkyl, C₅₋₁₂aryl, C₅₋₁₂ hetaryl wherein said aryl andhetaryl substituents may be further substituted by the non-aryl andnon-hetaryl substituents listed for phenyl ring A, and a protectedderivative of any thereof.

R² in the compound of formula (II) is selected from hydrogen, C₁₋₁₀alkyl(more suitably C₁₋₆alkyl, yet more suitably methyl), C₁₋₁₀haloalkyl(more suitably C₁₋₆haloalkyl such as C₁₋₆fluoroalkyl, for exampletrifluoromethyl), C₆₋₁₄aryl (suitably phenyl), C₆₋₁₄arylalkyl (suitablyphenyl-C₁₋₄alkyl, for example benzyl), and —(CH₂CH₂O)_(q)—CH₃ wherein qis an integer of from 1 to 10. Compounds of formula (II) wherein R² isC₄₋₁₀alkyl or —(CH₂CH₂O)_(q)—CH₃ wherein q is an integer of from 1 to 10may be used in the process where it is desirable to increase solubilityof the compound of formula (II) and as such these compounds and theprocess according to the invention using them, form separate aspects ofthe invention. Preferably, R² in the compound of formula (II) isselected from hydrogen and C₁₋₆alkyl, more preferably, R² is hydrogen.

R³ in the compound of formula (II) is a leaving group, capable of beingdisplaced by fluoride and is suitably selected from

-   nitro,-   —N₂ ⁺,-   chloro,-   bromo,-   iodo,-   —NR⁴(C₁₋₆alkyl)₂ ⁺ wherein R⁴ is C₁₋₆alkyl or a group of formula    (X):

-   —OSO₂R⁵ wherein R⁵ is selected from C₁₋₆alkyl, C₁₋₆haloalkyl such as    C₁₋₆perfluoroalkyl, aryl such as phenyl or tolyl (for example,    para-tolyl), and a group of formula (X) as defined above; and

wherein R⁶ is selected from hydrogen, C₁₋₆alkyl, halo, nitro, and agroup of formula (X) as defined above.

R³ in the compound of formula (II) is suitably selected from:

-   nitro,-   —N₂ ⁺,-   chloro,-   bromo,-   iodo,-   —NR⁴(C₁₋₆alkyl)₂ ⁺ wherein R⁴ is C₁₋₆alkyl;-   —OSO₂R⁵ wherein R⁵ is selected from C₁₋₆alkyl, C₁₋₆haloalkyl such as    C₁₋₆perfluoroalkyl, aryl such as phenyl or tolyl (for example,    para-tolyl); and

wherein R⁶ is selected from hydrogen, C₁₋₆alkyl, halo, and nitro.

In one particular aspect of the invention, R³ is nitro.

In the process according to the invention, use of compounds of formula(II) in which R³ comprises a group of formula (X) allows thefluorination to be performed in solid-phase which may simplifypurification of the fluorinated product as any unreacted precursorremains bound to the solid-support and may be removed from thesolution-phase product by filtration.

In the group of formula (X), the Solid Support may be any suitablesolid-phase support which is insoluble in any solvents to be used in theprocess but to which the Linker and/or compound of formula (II) can becovalently bound. Examples of suitable Solid Support include polymerssuch as polystyrene (which may be block grafted, for example withpolyethylene glycol), polyacrylamide, or polypropylene or glass orsilicon coated with such a polymer. The Solid Support may be in the formof small discrete particles such as beads or pins, or as a coating onthe inner surface of a cartridge or on a microfabricated vessel.

In the group of formula (X), the Linker may be any suitable organicgroup which serves to space the reactive site sufficiently from thesolid support structure so as to maximise reactivity. Suitably, theLinker comprises zero to four aryl groups (suitably phenyl) and/or aC₁₋₆ alkyl or C₁₋₆haloalkyl (suitably C₁₋₆ fluoroalkyl), and optionallyone to four additional functional groups such as amide or sulphonamidegroups. Examples of such linkers are well known to those skilled in theart of solid-phase chemistry, but include:

wherein at each occurrence, n is an integer of 0 to 3 and R^(L) ishydrogen or C₁₋₆alkyl.

Step (I) of the process according to the invention i.e. reaction of acompound of formula (II) with fluoride, suitably [¹⁸F]fluoride may beeffected using a fluoride source such as NaF, KF, CsF,tetraalkylammonium fluoride, or tetraalkylphosphonium fluoride, suitablyan [¹⁸F]fluoride source such as Na¹⁸F, K¹⁸F, Cs¹⁸F, tetraalkylammonium[¹⁸F]fluoride (for example, tetrabutylammonium [¹⁸F]fluoride), ortetraalkylphosphonium ¹⁸F fluoride. To increase the reactivity of thefluoride, a phase transfer catalyst such as an aminopolyether or crownether, for example, 4,7,13,16,21,24hexaoxa-1,10-diazabicyclo[8,8,8]hexacosane (Kryptofix 2.2.2) may beadded and the reaction performed in a suitable solvent. These conditionsgive reactive fluoride ions. Optionally, a free radical trap may be usedto improve fluoridation yields, as described in WO 2005/061415. The term“free radical trap” is defined as any agent that interacts with freeradicals and inactivates them. A suitable free radical trap for thispurpose may be selected from 2,2,6,6-Tetramethylpiperidine-N-Oxide(TEMPO), 1,2-diphenylethylene (DPE), ascorbate, para-amino benzoic acid(PABA), α-tocopherol, hydroquinone, di-t-butyl phenol, β-carotene andgentisic acid. Preferred free radical traps for use in the process ofthe invention are TEMPO and DPE, with TEMPO being most preferred.

The treatment with fluoride, suitably [¹⁸F]fluoride in step (i) may beeffected in the presence of a suitable organic solvent such asacetonitrile, dimethylformamide, dimethylsulphoxide, dimethylacetamide,tetrahydrofuran, dioxan, 1,2 dimethoxyethane, sulpholane,N-methylpyrolidininone, or in an ionic liquid such as an imidazoliumderivative (for example 1-ethyl-3-methylimidazoliumhexafluorophosphate), a pyridinium derivative (for example,1-butyl-4-methylpyridinium tetrafluoroborate), a phosphonium compound,or tetralkylammonium compound at a non-extreme temperature, for example,15° C. to 180° C., preferably at elevated temperature, such as 80° C. to150° C., for example around 120° C. The organic solvent is suitablyanhydrous, but in some cases may contain low levels of water.

In one aspect of the invention, the fluoro group in the compound offormula (I) is [¹⁸F]fluoro and the fluoride used in step (i) of theprocess is [¹⁸F]fluoride. There is particular need for novelradiofluoridation methods, especially for radiofluoridation of electronrich aromatic systems.

Step (ii) in the process, i.e. conversion of group —C(O)R² to hydrogen,is suitably effected by acid or base hydrolysis, using an organic orinorganic acid, at non-extreme temperature, for example ambienttemperature to reflux temperature. The reaction may be performed inpresence of an aqueous solvent or organic solvent, for exampleC₁₋₄alcohol such as methanol or ethanol or acetonitrile, or a mixture ofaqueous and organic solvents.

Suitable acids used in step (ii) include hydrobromic, trifluoroacetic,phosphoric, and hydrochloric.

Suitable bases used in step (ii) include sodium hydroxide or potassiumhydroxide. Use of sodium hydroxide in an organic solvent such asacetonitrile, at elevated temperature for example around 100° C. maygive rise to good radiochemical yields and facilitate purification ofthe fluorinated product.

Alternative bases used in step (ii) include non-nucleophilic bases suchas sodium hydride. This method gives rise to good radiochemical yieldsand also facilitates purification of the fluorinated product. Treatmentwith sodium hydride may be performed in a suitable aprotic solvent suchas acetonitrile or propionitrile and at elevated temperature such as 40°C. to 120° C., typically around 100° C. Sodium borohydride, lithiumborohydride, and lithium aluminium hydride are also suitable bases foruse in step (ii).

As would be apparent to the person skilled in the art, it is sometimesnecessary to use protecting group strategies to prevent unwantedside-reactions during organic synthesis. Examples of such strategies maybe found in Protecting Groups in Organic Synthesis, Theodora W. Greeneand Peter G. M. Wuts, published by John Wiley & Sons Inc. whichdescribes methods for incorporating and removing protecting groups. Toavoid unnecessary synthetic steps, it is particularly beneficial if anyprotecting groups remaining in the compound of formula (III) are removedunder the conditions of step (ii) so as to avoid a separate deprotectionstep.

As used herein, the term “alkyl” used alone or as part of another groupmeans a saturated straight or branched chain hydrocarbon radical, suchas methyl, ethyl, propyl, iso-propyl, butyl, iso-butyl, tert-butyl,n-pentyl, or n-hexyl.

As used herein, the term “alkenyl” used alone or as part of anothergroup means an unsaturated straight or branched chain hydrocarbonradical comprising at least one carbon to carbon double bond, such asethenyl, propenyl, iso-propenyl, butenyl, iso-butenyl, tert-butenyl,n-pentenyl, and n-hexenyl.

As used herein, the term “alkynyl” used alone or as part of anothergroup means an unsaturated straight or branched chain hydrocarbonradical comprising at least one carbon to carbon triple bond, such asethynyl, propynyl, iso-propynyl, butynyl, iso-butynyl, tert-butynyl,n-pentynyl, and n-hexynyl.

As used herein, the term “halo” used alone or as part of another groupmeans fluoro, chloro, bromo, or iodo.

As used herein, the term “aryl” used alone or as part of another groupmeans an aromatic hydrocarbon single ring or fused ring system, such asphenyl or naphthyl.

As used herein, the term “hetaryl” used alone or as part of anothergroup means an aromatic hydrocarbon single ring or fused ring systemwhich additionally comprises 1 or more heteroatoms selected fromsulphur, nitrogen, and oxygen, such as pyridyl, thiophenyl,benzothiazolyl, benzoxazolyl, or furyl.

The process according to the invention has particular utility forsynthesis of compounds of formula (I) in which R¹ is C₁₋₆alkyl, andparticularly methyl, thus the processes wherein R¹ in the compounds offormula (I), (II), (III) is C₁₋₆alkyl, and particularly methyl form aseparate aspect of the invention.

The process according to the invention is particularly useful when thefluoro group in the compound of formula (I) is ortho or para to thegroup —N(R¹)C(O)R² as the —N(R¹)C(O)R² positioned ortho or para to R³ inthe corresponding compound of formula (II) may activate R³ tonucleophilic displacement by fluoride. Preferably, the fluoro group inthe compound of formula (I) is ortho to the group —N(R¹)C(O)R² and thegroup R³ in the corresponding compound of formula (II) is ortho to thegroup —N(R¹)C(O)R².

Certain compounds of formula (I) are known to have use in diagnostic andtherapeutic methods, for example the benzothiazole derivatives describedfor in vivo imaging of amyloid according to the methods described in WO02/16333 and WO2004/083195. The methods previously described forpreparing these benzothiazole derivatives, although suitable forpreparing small amounts of the compounds, suffer from poor radiochemicalyields and poor reproducibility such that there exists a need forimproved processes for their preparation, particularly for a commercialsetting. As mentioned above, nucleophilic fluorination of an aromaticring can be problematic where the ring is electron rich. In thecompounds of formula (Ia) and (Ib) below, the substitution pattern makesthe aromatic ring difficult to fluorinate. Attempts to design a suitableprecursor for fluorination, which was stable, could be fluorinated ingood yield, and could then be readily converted to final product, wereproblematic as demonstrated below in Example 3. Thus, in a furtheraspect of the invention, there is provided a process for preparation ofa compound of formula (Ia):

whereinR¹ is selected from C₁₋₆alkyl, C₂₋₆alkenyl, and C₂₋₆alkynyl;R⁷, R⁸, R⁹, and R¹⁰ are each independently selected from hydrogen,fluoro, chloro, bromo, iodo, C₁₋₆alkyl, C₂₋₆alkenyl, C₂₋₆alkynyl,(CH₂)_(m)OR¹¹ (wherein m=1, 2, or 3), CF₃, CH₂—CH₂Y, O—CH₂—CH₂Y,CH₂—CH₂—CH₂Y, O—CH₂—CH₂—CH₂Y (wherein Y is selected from fluoro, chloro,bromo, and iodo), CN, (C═O)—R¹¹, N(R¹¹)₂, NO₂, (C═O)N(R¹¹)₂, O(CO) R¹¹,OR¹¹, SR¹¹, COOR¹¹, R_(ph), CR¹¹═CR¹¹—R_(ph), CR¹¹ ₂—CR¹¹ ₂—R_(ph)(wherein R_(ph) represents an unsubstituted or substituted phenyl groupwith the phenyl substituents being chosen from any of the non-phenylsubstituents defined for R⁷ to R¹⁰ and wherein R¹¹ is H or C₁₋₆alkyl)and a protected derivative of any thereof; andphenyl ring A is substituted by 1 to 3 substituents being chosen fromany of the non-phenyl substituents defined for R⁷ to R¹⁰;which comprises:(i) reaction of a corresponding compound of formula (IIa):

whereinR¹ is defined for the compound of formula (Ia),phenyl ring A is substituted as defined for the compound of formula(Ia); andR² is selected from hydrogen, C₁₋₁₀alkyl, C₁₋₁₀haloalkyl, C₆₋₁₄aryl,C₆₋₁₄arylalkyl, —(CH₂CH₂O)_(q)—CH₃ wherein q is an integer of from 1 to10;R³ is a leaving group as defined for the compound of formula (II);R⁷, R⁸, R⁹, and R¹⁰ are as defined for the compound of formula (Ia);with fluoride to give a compound of formula (IIIa)

wherein R¹ and R² are as defined for the compound of formula (IIa),phenyl ring A is substituted as defined for the compound of formula(Ia);R⁷, R⁸, R⁹, and R¹⁰ are as defined for the compound of formula (Ia);followed by step (ii) and optionally step (iii) in any order(ii) conversion of group —C(O)R² to hydrogen, suitably by hydrolysis(iii) removal of any further protecting groups.

In the compounds of formula (Ia), (IIa), and (IIIa) and thecorresponding process according to the invention, R⁷, R⁸, R⁹, and R¹⁰are suitably selected from hydrogen, hydroxy, —NO₂, —CN, —COOR¹¹,—OCH₂OR¹¹ (wherein R¹¹ is selected from hydrogen and C₁₋₆alkyl),C₁₋₆alkyl, C₂₋₆alkenyl, C₂₋₆alkynyl, C₁₋₆alkoxy, halo, and a protectedderivative of any thereof. Suitable protected derivatives ofsubstituents R⁷, R⁸, R⁹, and R¹⁰ would be apparent to the person skilledin the art, and are described in Theodora W. Greene and Peter G. M. Wutsas referenced hereinbefore. For example, when R⁷, R⁸, R⁹, or R¹⁰ ishydroxy, the hydroxy function is suitably protected as aC₁₋₆alkoxymethoxy group such as ethoxymethoxy or methoxymethoxy.

One class of preferred compounds of formula (Ia) for use in in vivoimaging of amyloid are those of formula (Ib)

wherein R¹ is selected from C₁₋₆alkyl, C₂₋₆alkenyl, and C₂₋₆alkynyl; andR⁹ is selected from hydroxy, —NO₂, —CN, —COOR¹¹, —OCH₂OR¹¹ (wherein R¹¹is selected from hydrogen and C₁₋₆alkyl), C₁₋₆alkyl, C₂₋₆alkenyl,C₂₋₆alkynyl, C₁₋₆alkoxy, halo, and a protected derivative of anythereof, and is preferably selected from hydroxy, C₁₋₆alkoxy, and aprotected derivative of any thereof, and is more preferably selectedfrom hydroxy, methoxy, and a protected derivative of either thereof.Therefore, according to one preferred aspect of the invention there isprovided a process for preparation of a compound of formula (Ib):

whereinR¹ is selected from C₁₋₆alkyl, C₂₋₆alkenyl, and C₂₋₆alkynyl;R⁹ is selected from hydroxy, —NO₂, —CN, —COOR, —OCH₂OR, C₁₋₆alkyl,C₂₋₆alkenyl, C₂₋₆alkynyl, C₁₋₆alkoxy, halo, and a protected derivativeof any thereof, and is preferably selected from hydroxy, C₁₋₆alkoxy, anda protected derivative of any thereof, and is more preferably selectedfrom hydroxy, methoxy, and a protected derivative of either thereof;which comprises:(i) reaction of a corresponding compound of formula (IIb):

wherein R¹ is selected from C₁₋₆alkyl, C₂₋₆alkenyl, and C₂₋₆alkynyl, andR² is selected from hydrogen, C₁₋₁₀alkyl, C₁₋₁₀haloalkyl, C₆₋₁₄aryl,C₆₋₁₄arylalkyl, —(CH₂CH₂O)_(q)—CH₃ wherein q is an integer of from 1 to10;R³ is a leaving group as defined for the compound of formula (II);R⁹ is as defined for the compound of formula (Ib);with fluoride to give a compound of formula (IIIb)

wherein R¹ and R² are as defined for the compound of formula (IIb);R⁹ is as defined for the compound of formula (Ib); followed by step (ii)and optionally step (iii) in any order(ii) conversion of group —C(O)R² to hydrogen, suitably by hydrolysis(iii) removal of any further protecting groups in substituent R⁹.

Compounds of formulae (IIa) and (IIb) as defined above are importantprecursors, useful for the preparation of in vivo imaging agents andtherefore form further aspects of the invention.

Preferred precursors of formula (IIa) and (IIIb) include those where R²is hydrogen or C₁₋₆alkyl, suitably methyl, more suitably R² is hydrogen;R¹ is C₁₋₆alkyl, suitably methyl; of these precursors in which R³ isnitro may have particular utility. Compounds of formula (IIa) and (IIb)in which R⁹ is hydroxy or C₁₋₆alkoxy or a protected derivative thereofmay also have particular utility. One preferred such precursor is2-[3-nitro-4-(methylformylamino)phenyl]-6-ethoxymethoxy-benzothiazole.

Conveniently, a precursor of formula (II), (IIa), or (IIb) could beprovided as part of a kit, for example for use in a radiopharmacy. Thekit may contain a cartridge which can be plugged into a suitably adaptedautomated synthesiser. The cartridge may contain, apart from theprecursor, a column to remove unwanted fluoride ion, and an appropriatevessel connected so as to allow the reaction mixture to be evaporatedand allow the product to be formulated as required. The reagents andsolvents and other consumables required for the synthesis may also beincluded together with a compact disc carrying the software which allowsthe synthesiser to be operated in a way so as to meet the customersrequirements for radioactive concentration, volumes, time of deliveryetc. Conveniently, all components of the kit are disposable to minimisethe possibilities of contamination between runs and may be sterile andquality assured.

The invention further provides a radiopharmaceutical kit for thepreparation of an ¹⁸F-labelled tracer for use in PET, which comprises:

-   (i) a vessel containing a compound of formula (II), (IIa), or (IIb);    and-   (ii) means for eluting the vessel with a source of ¹⁸F⁻;-   (iii) an ion-exchange cartridge for removal of excess ¹⁸F⁻; and    optionally-   (iv) a cartridge for deprotection of the resultant product of    formula (I), (Ia), or (Ib).

The invention further provides a cartridge for a radiopharmaceutical kitfor the preparation of an ¹⁸F-labelled tracer for use in PET whichcomprises:

(i) a vessel containing a compound of formula (II), (IIa), or (IIb); and

(ii) means for eluting the vessel with a source of ¹⁸F⁻.

Compounds of formula (II), (IIa), and (IIb) may be prepared fromcommercially available starting materials or using starting materialsdescribed in WO 02/16333 and WO2004/083195, by standard methods oforganic chemistry, for example by the methods described below and in theexamples.

Compounds of formula (II), (IIa), and (IIb) wherein R³ is nitro may beprepared by methods analogous to those described in Example 1.

Compounds of formula (II), (IIa), and (IIb) wherein R³ is chloro, bromo,iodo, tosylate, or an iodonium salt, may be prepared by methodsanalogous to those shown in Schemes 1 to 4 respectively.

In Schemes 1 to 6, R² is as defined for a compound of formula (I) above,R in scheme 1 is an alkyl or aryl substituent, Ac is acyl, Ts is tosyl,NaHDMS is sodium hexamethyldisilazide, TFA is trifluoroacetic acid,Pd₂dba₃ in Scheme 6 is tris-(dibenzylideneacetone)dipalladium(0) and theother abbreviations are as defined in the Examples.

Compounds of formula (II), (IIa), and (IIb) wherein R³ is —N₂ ⁺ may beprepared from the corresponding compound wherein R³ is nitro, byreduction of the nitro group to amino, for example using hydrogen andPd/C as catalyst and then diazotization using NaNO₂.

Compounds of formula (II), (IIa), and (IIb) wherein R³ is—NR⁴(C₁₋₆alkyl)₂ ⁺ may be prepared according to Scheme 6.

The invention is now illustrated by way of the following Examples, inwhich the following abbreviations are used:

DMF: N,N-dimethylformamide

DCM: dichloromethane

EOMCl: ethoxymethoxychloride

DMAP: dimethylaminopyridine

RT: room temperature

THF: tetrahydrofuran

IMS: industrial methylated spirits

M.p.: melting point

eq.: equivalents

EtOAc: ethyl acetate

QMA: quaternary ammonium

HPLC: high performance liquid chromatography

mL or ml: milliliter(s)

TLC: thin layer chromatography

v/v: volume/volume

NMR: nuclear magnetic resonance

MS: mass spectrometry

EXAMPLE 1 Synthesis of2-[3-nitro-4-(methylformylamino)phenyl]-6-ethoxymethoxybenzothiazole

Example 1(i) 4-Acetamido-3-nitrobenzoyl chloride (2)

4-Acetamido-3-nitrobenzoic acid 1 (Alfa Aesar, 5.6 g, 25 mmol), oxalylchloride (4.76 g, 38 mmol), chloroform (50 ml), DMF (few drops) werestirred at 40° C. for 3 hours. The solvent was removed in vacuo to givea yellow solid that was used in the next step without furtherpurification.

Example 1(ia) 5-methoxy-2-aminobenzenethiol (3)

2-Amino-6-methoxy-benzothiazole 10 g (55.6 mmol) was suspended in 25%aqueous potassium hydroxide and the mixture heated under reflux for 24h. The pale yellow solution was cooled and acidified to pH 6 withfirstly aqueous 6N HCl then acetic acid. The precipitated solid wasfiltered, washed with water (3×100 ml), dried (high vac.) to afford thedesired material as a pale yellow powder 8.18 g, 95%.

Example 1(ii) 2-(4-Acetamido-3-nitrophenyl)-6-methoxybenzothiazole (4)

5-Methoxy-2-aminobenzenethiol 3 (3.88 g, 25 mmol), pyridine (100 ml),and DMAP (few crystals) were stirred at room temperature.

4-Acetamido-3-nitrobenzoyl chloride (25 mmol, as produced above) wasadded in one portion below 30° C. The mixture was stirred for a further1 hour. The mixture was heated to 80° C. and stirred over the weekend.The mixture was cooled. The crystals were filtered off and washed withIMS to give 2.2 g (26% yield) of2-(4-acetamido-3-nitrophenyl)-6-methoxybenzothiazole.

Example 1(iii)2-(4-N-Methylacetamido-3-nitrophenyl)-6-methoxybenzothiazole (5)

Sodium hydride (6.33 g, 157 mmol) and DMF (400 ml) were stirred at roomtemperature. 2-(4-Acetamido-3-nitrophenyl)-6-methoxybenzothiazole 4 (45g, 131 mmol) was added in one portion. The mixture was stirred for 1hour. The mixture was cooled in an ice bath and methyl iodide (23.1 g,164 mmol) was added in one portion, the temperature stayed below 20° C.

The mixture was stirred for 3 hours, water (900 ml) was added, themixture was filtered and washed with water. The solid was recrystallisedfrom IMS to give 43.7 g (93% yield) of2-(4-N-methylacetamido-3-nitrophenyl)-6-methoxybenzothiazole. M.p168-172° C.

Example 1(iv) 2-(4-Methylamino-3-nitrophenyl)-6-hydroxybenzothiazole (6)

A mixture of2-(4-N-methylacetamido-3-nitrophenyl)-6-methoxybenzothiazole (58 g, 162mmol), hydrobromic acid (500 ml, 48% aqueous solution) and hydrobromicacid (500 ml, 45% in acetic acid) were stirred at 135° C. for 5 hr. Themixture was cooled to room temperature and the solid was filtered offand washed with a little water. The solid was slurried with water andthe pH adjusted to about pH10 with concentrated ammonia solution. Thesolid was filtered off and washed with water.

The solid was triturated with IMS (200 ml), filtered, the mixture wasboiled with IMS (500 ml) then cooled to room temperature then filtered.The solid was again boiled with IMS (500 ml) then cooled to roomtemperature then filtered. The solid was dissolved in hot DMF (200 ml),filtered and water (100 ml) added. The solid was filtered off and washedwith IMS. The solid was boiled with water (300 ml) for 5 minutes,cooled, filtered, washed with water then IMS to give 45.9 g (94% yield)of 2-(4-Methylamino-3-nitrophenyl)-6-hydroxybenzothiazole. M.p 269-272°C.

Example 1(v)2-[3-nitro-4-(methylamino)phenyl]-6-ethoxymethoxybenzothiazole (7)

A 3 necked 250 ml round bottom flask was dried in an oven at 80° C.overnight. A suspension of 6 (16.6 mmol, 5 g) in dry THF (180 ml) hasbeen poured drop wise into a suspension of NaH 60% dispersion in mineraloil (33.2 mmol, 1.26 g, 2 eq) in dry THF (20 ml). Once the addition wascomplete neat ethoxy methyl chloride (16.6 mmol, 1.54 ml, 1 eq) wasadded and the reaction stirred over night. The dark brown mixture wasfiltrated under vacuum and the filtrate concentrated under high vacuum.

The crude product was supported onto silica and purified via flashchromatography in DCM/EtOAc: 3% EtOAc

The desired fraction was isolated, concentrated under high vacuum toyield 60% of an off-red solid with 95% purity.

Example 1(vi)2-[3-nitro-4-(methylformylamino)phenyl]-6-ethoxymethoxy-benzothiazole(8)

All glassware was dried in an oven at 80° C. overnight.

In a 1 L three necked round bottom flask fitted with a condenser andthermometer was added acetic anhydride (15 ml, 160 mmol, 22 eq) dropwiseto a solution of formic acid (160 mmol, 6 ml, 22 eq) at 0° C. Themixture was stirred for 15 minutes at 60° C.

A solution of 7 (7.2 mmol, 2.6 g) in dry DCM (310 ml) is added dropwiseat 0° C. to the mixed anhydride. Stirring was continued at thistemperature for one hour and the clear orange solution was stirred 5days at 40° C.

The reaction was followed by HPLC: after 5 days 60% conversion into thedesired product was observed.

HPLC Conditions:

Column Phenomenex Luna 150×4.6 mn

Flow 1 ml/minutes

Solvent: Acetonitrile (B) and Water (A)

Detection: 254-214

Gradient 5-95% of B over 8 minutes

Retention time: 9.5 minutes

The clear orange solution was washed with 1N NaOH aq (3×100 ml), water(3×100 ml), dried over magnesium sulfate and concentrated under highvacuum.

The bright orange crude product was supported onto silica and purifiedvia flash chromatography in DCM/EtOAc: 3-10% EtOAc

The desired fraction was isolated, concentrated under high vacuum toyield 54.2% of an off yellow solid with 98% purity.

Example 1(vii) Preparation of2-[3-[¹⁸F]fluoro-4-(methylamino)phenyl]-6-hydroxy-benzothiazole (11)—SeeScheme Below (Approach 1)

[¹⁸F]fluoride (in 200 μL enriched 95% ¹⁸O water), 2.5 mg of Kryptofix2.2.2 (in 0.5 mL acetonitrile) and 50 μL 0.1M K₂CO₃ were added to aglassy carbon reaction vessel. The solution was then evaporated todryness using a stream of nitrogen and heating the reaction vessel to100° C. for 15 minutes. 2×1 mL acetonitrile was added to the reactionvessel at 5 minutes and 10 minutes respectively to aid azeotropicdrying. The reaction vessel was cooled to room temperature and2-[3-nitro-4-(methylformylamino)phenyl]-6-ethoxymethoxy-benzothiazole(8) (5.0 mg) in 1 mL anhydrous dimethyl sulfoxide was added. Thereaction was sealed and heated for 10 minutes at 130° C., The crudemixture was analyzed by HPLC and TLC.

0.25 mL of 6M HCl and 0.5 mL DMSO was added to the crude reactionsolution of2-[3-[¹⁸F]fluoro-4-(methylformylamino)phenyl]-6-ethoxymethoxy-benzothiazole(9) and heated at 125° C. for 10 minutes. The reaction was then cooledto room temperature and neutralized using 2M sodium acetate resulting inthe synthesis of2-[3-[¹⁸F]fluoro-4-(methylamino)phenyl]-6-hydroxy-benzothiazole (11).The crude mixture was analyzed by HPLC and TLC.

HPLC Purification and Formulation.

2-[3-[¹⁸F]fluoro-4-(methylamino)phenyl]-6-hydroxy-benzothiazole (11) waspurified by HPLC using a Phenomenex Prodigy ODS preparative. 10 um 250mm×10 mm (part no. 00G-4088-N0) preparative column (the column is elutedwith 40/60 acetonitrile/triethylamine phosphate buffered solution pH 7(v/v)). The control method is 0-15 minutes 5 ml/minute, 15.5-39.9minutes 8 ml/minute, 40 minutes 5 ml/minute. The product elutes with aretention time of 22-23 minutes (in a volume of 8 mL).

The HPLC purified “cut” was diluted to 50 mL with addition of distilledwater. The product was then “trapped” onto a C8-sep-pak cartridge andthen eluted off the cartridge with 1 mL ethanol. The ethanol was thenremoved under vacuum and the final product formulated in 10% ethanol/90%phosphate buffered saline.

EXAMPLE 2 Preparation of2-[3-[¹⁸F]fluoro-4-(methylamino)phenyl]-6-hydroxy-benzothiazole (11)—SeeScheme Above (Approach 2) Example 2(i) Preparation of [K/K2.2.2]⁺ ¹⁸F(Using Enriched 95% ¹⁸O Water)

After irradiation, the target content was passed through a column packedwith QMA resin. The column was purged with helium for 5 minutes. The[¹⁸F]fluoride adsorbed on the resin was eluted into a reaction vial with4 ml of a 96:4 (by volume) acetonitrile-water mixture containing 19.1 mgof Kryptofix 2.2.2 and 2.9 mg of K₂CO₃; the solution was then evaporatedand co-evaporated with anhydrous acetonitrile (2×1 ml) to dryness in anitrogen stream at 110° C.

Example 2(ii) Preparation of2-[3-[¹⁸F]fluoro-4-(methylformylamino)phenyl]-6-ethoxymethoxy-benzothiazole(9) and2-[3-[¹⁸F]fluoro-4-(methylamino)phenyl]-6-ethoxymethoxy-benzothiazole(10)

A solution of2-[3-nitro-4-(methylformylamino)phenyl]-6-ethoxymethoxy-benzothiazole(8) (3.0 mg) in anhydrous acetonitrile (0.1 ml) was added to a solutionof [K/K2.2.2]⁺ ¹⁸F⁻ in anhydrous acetonitrile (0.25 ml). The reactionmixture was heated at 150° C. for 15 minutes. The crude mixture wasanalyzed by analytical HPLC.

Example 2(iii) Conversion of2-[3-[¹⁸F]fluoro-4-(methylformylamino)phenyl]-6-ethoxymethoxy-benzothiazole(9) to2-[3-[¹⁸F]fluoro-4-(methylamino)phenyl]-6-ethoxymethoxy-benzothiazole(10)

About 0.2 ml of the previous reaction mixture was added to a solution ofNaH (3.2 mg) in anhydrous acetonitrile (0.2 ml) at room temperature. Theresulted mixture was heated at 100° C. for 5 minutes. The crude mixturewas analyzed by analytical HPLC.

Preparation of2-[3-[¹⁸F]fluoro-4-(methylamino)phenyl]-6-hydroxy-benzothiazole (11)

A solution of concentrated HCl in MeOH (1:2) (0.25 ml) was added to theprevious reaction mixture and heated at 100° C. for 5 minutes. The crudemixture was analyzed by analytical HPLC.

EXAMPLE 3 Comparative [¹⁸F]Fluorination of Different Precursors

Radiofluoridation of various benzothiazole precursor compounds usingmethods analogous to those described in Example 1(vii) gave the resultsshown in Table 1. Crude yield was calculated from radiochemical puritymeasured by HPLC, corrected for product lost by retention on HPLC and inthe reaction vessel.

Precursor; R* = H <5% crude Yield

<5% crude Yield

<5% crude Yield

<5% crude Yield

Current labeling 25-30% incorporation

EXAMPLE 4 Automated synthesis of2-[3-[¹⁸F]fluoro-4-(methylamino)phenyl]-6-hydroxy-benzothiazole (11)

The reagent positions of a TRACERIab FX_(FN) (GE Healthcare Ltd)automatic synthesiser unit were loaded with the following solutions:

-   -   i. 0.1M potassium carbonate in water (0.5 mL)    -   ii. 0.13M Kryptofix 2.2.2 in acetonitrile (0.5 mL)    -   iii. Precursor solution: 0.1M        2-[3-nitro-4-(methylformylamino)phenyl]-6-ethoxymethoxy-benzothiazole (8)        in DMSO (1.0 mL)    -   iv. 4M hydrochloric acid (0.25 mL)    -   v. ethanol (1.0 mL)    -   vi. 0.01M phosphate buffer, pH 7.4 (13.1 mL)

When a solution of [¹⁸F]fluoride in [¹⁸O]-enriched water (121 MBq) hadbeen loaded into the synthesiser's starting position the operatorinitiated the programme causing the following sequence of events to takeplace.

The fluoride solution passed through a QMA cartridge (pre-conditionedwith 10 mL of 0.5M aqueous potassium carbonate and 20 mL of water)trapping the fluoride and sending the enriched water to waste. The QMAcartridge was then eluted with the 0.1M potassium carbonate solution torecover the fluoride and the eluant was directed to the reactor vessel.The solution of Kryptofix 2.2.2 was added to the reactor and the mixturewas heated at 60° C. for 5 minutes under a gentle flow of nitrogen atreduced pressure. The temperature was then raised to 120° C. and heldunder vacuum for 7 minutes to dry the contents of the reactor. Aftercooling to 50° C., the Precursor solution was added to the reactor andthe temperature was raised to 135° C. for 10 minutes. This step allows[¹⁸F]fluoride to be incorporated into the organic molecule. The solutionwas cooled to 50° C. and the 4M hydrochloric acid was added. The mixturewas heated to 125° C. for 5 minutes to cause deprotection of theintermediate compound and, after cooling to 40° C., the crude productsolution was injected onto a Phenomenex Gemini C18 HPLC column (250×21.2mm, 5 μm). The column was eluted with 6 mM hydrochloricacid-acetonitrile mixture (53:47, v:v) at 10 mL/min. The desired productwas identified by radio-detection and collected by cutting. The obtainedsolution was diluted with water (150 mL) and passed through a Sep-Pak®Plus C8 cartridge (pre-conditioned with 10 mL ethanol and 10 mL water)so that the product was retained on the cartridge. The cartridge waseluted with ethanol into the product vial which contained propyleneglycol (0.9 mL). Phosphate buffer was also passed through the cartridgeinto the product vial to give the formulated product. Yield of theproduct was 10.8% (non-corrected, based on [¹⁸F] starting activity) andthe radiochemical purity was >99%.

EXAMPLE 5 Alternative synthesis of2-[3-nitro-4-(methylformylamino)phenyl]-6-ethoxymethoxy-benzothiazole(8) Example 5(i) Synthesis of4-Chloro-N-(4-hydroxy-phenyl)-3-nitro-benzamide

4-Amino-phenol (12 g, 0.11 mol, Acros and Aldrich) was dissolved underan inert atmosphere, with stirring, in dry DMF (50 ml) and cooled in anice-bath. Triethylamine (TEA, 11 g, 0.11 mol) was added and stirringcontinued for 1 hour. 4-Chloro-3-nitro-benzoyl chloride (22.2 g 0.1 mol,Acros and Aldrich) was added slowly and stirred overnight. Theprecipitated triethylamine.hydrochloride salt was filtered off and DMFremoved under reduced pressure. The residue was extracted with EtOAc(3×100 ml) and citric acid (1M, 3×100 ml). The organic phase was driedwith magnesium sulphate, filtered and evaporated to dryness underreduced pressure. The title product was recrystallised frommethanol/water (1:1, 250 ml), yield 85%, and analysed by NMR and MS.

Example 5(ii) Synthesis of4-Chloro-N-(4-ethoxymethoxy-phenyl)-3-nitro-benzamide

4-Chloro-N-(4-hydroxy-phenyl)-3-nitro-benzamide (14.6 g, 0.05 mol) wasplaced in an oven dried 2-necked 500 ml round-bottomed flask and flushedwith N₂. Enough Dimethoxyethane (DME, 100 ml) was added to dissolve theamide. The mixture was cooled in ice bath and sodium hydride (NaH, 50%in oil, 3.6 g total, 0.075 mol) added in small portions with vigorousstirring. An hour after addition was complete, chloromethoxy-ethane(7.13 g, 0.075 mol, commercially available) was added drop wise via apressure-equalising dropping funnel. The reaction was followed by TLC(dichloromethane, DCM,: methanol, MeOH, 95:5). The reaction mixture waspoured into ice water and extracted with EtOAc (3×50 ml). The organicphase was dried (MgSO₄) and evaporated under reduced pressure. The crudeproduct was recrystallised form Hexane/ethyl acetate, 1:4, to give 81%of the title compound.

Example 5(iii) Synthesis of4-Chloro-N-(4-ethoxymethoxy-phenyl)-3-nitro-thiobenzamide

4-Chloro-N-(4-ethoxymethoxy-phenyl)-3-nitro-benzamide (3.5 g, 10 mmol),phosphorus pentasulfide P₄S₁₀ (0.81 g, 1.83 mmol, commerciallyavailable), hexamethyldisiloxane (2.7 g, 16.7 mmol, commerciallyavailable) and toluene (10 ml) were added into a 100 ml round-bottomedflask and flushed with nitrogen. The mixture was heated under reflux andwas followed by TLC. Heating continued until no more starting benzamideremained. Microwave heating can also be used. The reaction mixture wascooled to room temperature. Potassium carbonate solution (4 ml of 5.3Msolution) was added. Acetone (10 ml) was added and the mixture stirredfor 1 hour in an ice bath, the extracted with toluene and water. Theorganic phase was dried (MgSO₄) and toluene removed under reducedpressure and purified by flash chromatography with ethyl acetate/hexaneas eluent.

Example 5 (iiia) Alternative Thioamidation procedure: synthesis ofN-(4-Benzyloxy-phenyl)-4-chloro-3-nitro-thiobenzamide

N-(4-Benzyloxy-phenyl)-4-chloro-3-nitro-benzamide (19.15 g, 50 mmol),Lawesson reagent (11 g, 27 mmol, commercially available) and dioxane(150 ml) were stirred together and heated under reflux for 4 h. When nomore starting amide was present, as showed by TLC, the reaction mixturewas cooled and the solvent removed under reduced pressure. The crudeproduct was dissolved in minimum boiling toluene to recrystallise. Thepurified product was filtered and washed with cold toluene and coldhexane to give the thioamide, in 77% yield.

Example 5(iva) Synthesis of6-benzyloxy-2-(4-chloro-3-nitro-phenyl)-benzothiazole

N-(4-Benzyloxy-phenyl)-4-chloro-3-nitro-thiobenzamide (2 g, 5 mmol) wasdissolved in methanol (100 ml). Sodium hydroxide (1.6 g in 5 ml water)was added followed by Triton B (2.1 ml, 5 mmol, commercially available).The mixture was cooled in an ice-bath. Potassium ferri(III)cyanide (13.2g in 50 ml water) was added drop wise with vigorous stirring. Thereaction mixture was allowed to warm up overnight and was further warmedto 130° C. for 1 hour. The reaction mixture was cooled and extractedwith ethyl acetate/water. The organic phase dried and the solventremoved under reduced pressure. The product, compound was purified byflash chromatography with Hexane/Ethyl acetate as eluent.

Example 5(iv) Synthesis of6-ethoxymethoxy-2-(4-chloro-3-nitro-phenyl)-benzothiazole

Using methods analogous to those described in Example 5(iva), thethiobenzamide prepared in Example 5(iii) may be cyclised to form thetitle compound.

Example 5(v) Synthesis of6-benzyloxy-2-(4-methylamino-3-nitro-phenyl)-benzothiazole and6-ethoxymethoxy-2-(4-methylamino-3-nitro-phenyl)-benzothiazole (7)

The compounds of Examples 5(iva) and 5(iv) respectively, are reactedwith methylamine in aqueous solution, heating to 130° C., for example ina microwave oven. The reaction mixture is extracted with ethylacetate/water and the organic phase is dried, before removing thesolvent under reduced pressure. The title products are purified by flashchromatography using hexane/ethyl acetate.

Example 5(vi) Synthesis of2-[3-nitro-4-(methylformylamino)phenyl]-6-benzyloxy-benzothiazole and2-[3-nitro-4-(methylformylamino)phenyl]-6-ethoxymethoxy-benzothiazole(8)

The title compounds are prepared from the compounds of Example 5(v)respectively using formylation methods analogous to those described inExample 1(vi).

EXAMPLE 6 Alternative synthesis of2-[3-nitro-4-(methylformylamino)phenyl]-6-ethoxymethoxy-benzothiazole(8)

The synthesis is performed, by analogy to Example 5, but starting with4-amino-3-chloro-phenol to prepare6-ethoxymethoxy-2-(4-chloro-3-nitro-phenyl)-benzothiazole via4-Chloro-N-(4-hydroxy-2-chloro-phenyl)-3-nitro-benzamide,4-Chloro-N-(4-ethoxymethoxy-2-chloro-phenyl)-3-nitro-benzamide, and4-Chloro-N-(4-ethoxymethoxy-2-chloro-phenyl)-3-nitro-thiobenzamide.Cyclisation of4-chloro-N-(4-ethoxymethoxy-2-chloro-phenyl)-3-nitro-thiobenzamide toform 6-ethoxymethoxy-2-(4-chloro-3-nitro-phenyl)-benzothiazole isperformed using literature methods, for example Bowman et alTetrahedron, 47(48), 10119-10128 (1991); Couture and Glandclaudon,Heterocycles, 22(6) 1984; Hutchinson et al, Tetrahedron Lett. 2000,41(3), 425-8. Subsequent methylation and formylation are then performedas described in Example 5.

The invention claimed is:
 1. A compound of formula (IIa),

wherein R¹ is C₁₋₆alkyl, C₂₋₆alkenyl, or C₂₋₆alkynyl; phenyl ring A is substituted by 1 to 3 substituents being chosen from any of the non-phenyl substituents defined for R⁷ to R¹⁰; R² is hydrogen, C₁₋₁₀alkyl, C₁₋₁₀haloalkyl, C₆₋₁₄aryl, C₆₋₁₄arylalkyl, or —(CH₂CH₂O)_(q)—CH₃ where q is an integer of from 1 to 10; R³ is nitro, —N₂ ⁺, chloro, bromo, iodo, —NR⁴(C₁₋₆alkyl)₂ ⁺ where R⁴ is C₁₋₆alkyl, —OSO₂R⁵

where R⁵ is C₁₋₆alkyl, C₁₋₆haloalkyl, or aryl, or where R⁶ is hydrogen, C₁₋₆alkyl, halo, aor nitro; R⁷, R⁸, R⁹, and R¹⁰ are each independently, fluoro, chloro, bromo, iodo, C₁₋₆alkyl, C₂₋₆alkenyl, C₂₋₆alkynyl, (CH₂)_(m)OR¹¹ (where m is 1, 2, or 3), CF₃, CH₂—CH₂Y, O—CH₂—CH₂Y, CH₂—CH₂—CH₂Y, O—CH₂—CH₂—CH₂Y (where Y is fluoro, chloro, bromo, or iodo), CN, (C═O)—R¹¹, N(R¹¹)₂, NO₂, (C═O)N(R¹¹)₂, O(CO) R¹¹, OR¹¹, SR¹¹, COOR¹¹, R_(ph), CR¹¹═CR¹¹—R_(ph), CR¹¹ ₂—CR¹¹ ₂—R_(ph) (where R_(ph) is an unsubstituted or substituted phenyl group with the phenyl substituents being chosen from any of the non-phenyl substituents defined for R⁷ to R¹⁰ and where R¹¹ is H or C₁₋₆alkyl) or a protected derivative of any thereof.
 2. A compound of formula (IIb):

wherein R¹ is C₁₋₆alkyl, C₂₋₆alkenyl, or C₂₋₆alkynyl; R² is hydrogen, C₁₋₁₀alkyl, C₁₋₁₀haloalkyl, C₆₋₁₄aryl, C₆₋₁₄arylalkyl, or —(CH₂CH₂O)_(q)—CH₃ where q is an integer of from 1 to 10; R³ is nitro, —N₂ ⁺, chloro, bromo, iodo, —NR(C₁₋₆alkyl)₂ ⁺ where R⁴ is C₁₋₆alkyl, —OSO₂R⁵

where R⁵ is C₁₋₆alkyl, C₁₋₆haloalkyl, aryl, or where R⁶ is hydrogen, C₁₋₆alkyl, halo, or nitro; R⁹ is hydroxy, —NO₂, —CN, —COOR¹¹ or —OCH₂OR¹¹ (where R¹¹ is hydrogen or C₁₋₆alkyl), C₁₋₆alkyl, C₂₋₆alkenyl, C₂₋₆alkynyl, C₁₋₆alkoxy, or halo, or a protected derivative of any thereof.
 3. A compound of formula (IIb) according to claim 2 where R² is hydrogen and R³ is nitro.
 4. A compound 2-[3-nitro-4-(methylformylamino)phenyl]-6-ethoxymethoxy-benzothiazole.
 5. A radiopharmaceutical kit for the preparation of an ¹⁸F-labelled tracer for use in PET, which comprises: (i) a vessel containing a compound of formula (IIa), as defined in claim 1; (ii) means for eluting the vessel with a source of ¹⁸F⁻; (iii) an ion-exchange cartridge for removal of excess ¹⁸F; and optionally (iv) a cartridge for deprotection of the resultant product of formula (Ia):

wherein R¹ is C₁₋₆alkyl, C₂₋₆alkenyl, or C₂₋₆alkynyl; R⁷, R⁸, R⁹, and R¹⁰ are each independently, fluoro, chloro, bromo, iodo, C₁₋₆alkyl, C₂₋₆alkenyl, C₂₋₆alkynyl, (CH₂)_(m)OR¹¹ (where m is 1, 2, or 3), CF₃, CH₂—CH₂Y, O—CH₂—CH₂Y, CH₂—CH₂—CH₂Y, O—CH₂—CH₂—CH₂Y (where Y is fluoro, chloro, bromo, or iodo), CN, (C═O)—R¹¹, N(R¹¹)₂, NO₂, (C═O)N(R¹¹)₂, O(CO) R¹¹, OR¹¹, SR¹¹, COOR¹¹, R_(ph), CR¹¹═CR¹¹—R_(ph), CR¹¹ ₂—CR¹¹ ₂—R_(ph) (where R_(ph) is an unsubstituted or substituted phenyl group with the phenyl substituents being chosen from any of the non-phenyl substituents defined for R⁷ to R¹⁰ and where R¹¹ is H or C₁₋₆alkyl) or a protected derivative of any thereof; and phenyl ring A is substituted by 1 to 3 substituents being chosen from any of the non-phenyl substituents defined for R⁷ to R¹⁰.
 6. A cartridge for a radiopharmaceutical kit for the preparation of an ¹⁸F⁻ labelled tracer for use in PET which comprises: (i) a vessel containing a compound of formula (IIa) as defined in claim 1; and (ii) means for eluting the vessel with a source of ¹⁸F⁻.
 7. A radiopharmaceutical kit for the preparation of an ¹⁸F⁻ labelled tracer for use in PET, which comprises: (i) a vessel containing a compound of formula (IIb) as defined in claim 2; (ii) means for eluting the vessel with a source of ¹⁸F⁻; (iii) an ion-exchange cartridge for removal of excess ¹⁸F⁻; and optionally (iv) a cartridge for deprotection of the resultant product of formula (Ib)

wherein R¹ is C₁₋₆alkyl, C₂₋₆alkenyl, or C₂₋₆alkynyl; R⁹ is hydroxy, —NO₂, —CN, —COOR¹¹ or —OCH₂OR¹¹ (where R¹¹ is hydrogen or C₁₋₆alkyl), C₁₋₆alkyl, C₂₋₆alkenyl, C₂₋₆alkynyl, C₁₋₆alkoxy, or halo, or a protected derivative of any thereof.
 8. A cartridge for a radiopharmaceutical kit for the preparation of an ¹⁸F⁻ labelled tracer for use in PET which comprises: (i) a vessel containing a compound of formula (IIb) as defined in claim 2; and (ii) means for eluting the vessel with a source of ¹⁸F⁻. 